1. Field of the Invention
The present invention is directed to technology and designs of efficient acoustic (sonic and ultrasonic) bulk wave transducers for simultaneous operation in at least two frequency bands. Applications of the transducers are for example, but not limited to: medical ultrasound imaging, nondestructive testing, industrial and biological inspections, geological applications, and SONAR applications.
2. Description of the Related Art
The utilization of the nonlinear elasticity of tissue and ultrasound contrast agent micro-bubbles in medical ultrasound imaging provides improved images with less noise. The widest use is in the so-called harmonic imaging, where the 2nd harmonic component of the transmitted frequency band is used for the imaging. A use of 3rd and 4th harmonic components of the transmitted pulse for imaging is also presented in U.S. Pat. No. 6,461,303.
Dual band transmitted pulses were used in M-mode and Doppler measurements in Br Heart J. 1984 Jan;51(1):61-9. Further examples are shown in U.S. Pat. No. 5,410,516 where sum and difference bands of the transmitted bands produced in the nonlinear scattering from contrast agent micro-bubbles where detected. A further development of this dual band transmission is done in U.S. Pat. No. 6,312,383 and U.S. patent application Ser. No. 10/864,992.
U.S. patent application Ser. Nos. 10/189,350 and 10/204,350 describe in depth different uses of dual band transmitted ultrasound and acoustic pulse complexes that provide images with reduced noise, images of nonlinear scattering, and quantitative object parameters that greatly enhance the use of ultrasound and acoustic imaging. The methods are applicable both with transmission and back scatter imaging. For these applications one would transmit dual band pulse complexes as illustrated by example in FIG. 1, where in FIG. 1a a high frequency (HF) pulse 101 rides on the peak pressure of a low frequency (LF) pulse 102. FIG. 1b shows another situation where the HF pulse 103 rides on the maximal gradient of the LF pulse 102. The ratios of the center frequencies of the LF and HF pulses can typically be in the range of 1:5-1:20, and at the same time the HF pulse must be found in defined intervals of the LF pulse throughout defined depth ranges of the images. These requirements provide the following two challenges for the design of the transducer arrays:
1. For the HF pulse to propagate within a range interval of the LF pulse throughout a depth range of the image, the HF and LF radiation surfaces must have a large common area. One generally wants pulse pressures in the HF and the LF pulses in the order of MPa, depending on the application. This requires high electro to acoustic transfer efficiency which with current transducer technology is obtained with resonant operation of the transduction, which in turn gives a limited operative frequency band of current ultrasound transducer arrays, which do not cover the required frequency band (typically 1:5-1:15) for the described applications. Because one needs a large part of the HF and LF radiation surfaces to be common, this wide band presents special challenges for the structural vibration design of the arrays.
2. The large frequency separation between the HF and the LF pulses means that the wave length of the LF pulses is much larger than the wave length of the HF pulses, typically by a factor 5-15, which means that the LF pulse beam is subject to much higher diffractive beam divergence than the HF pulse beam. For adequate collimation of the LF beam to maintain high LF pressures at large depths, one therefore in certain applications needs to use larger width of the LF aperture than the HF aperture. This produces a sliding of the position of the HF pulse relative to the LF pulse with propagation distance, which sets special requirements for the design of the dual frequency radiation surfaces.
In another application one wants from the same probe to transmit a low frequency (e.g. 0.5-2 MHz) wave for treatment (hyperthermia or cavitation destruction of tissue), while being able to provide ultrasound imaging from the same probe surface at a higher frequency (e.g. 5-10 MHz). In yet other applications one simply wants to have a larger frequency band available for imaging from the same probe at a large variation of depth ranges.
The current invention presents several solutions to these challenges of transducer designs that can transmit/receive dual band ultrasound pulse complexes with large separation between the low and high frequencies, and with limited position/phase sliding between the transmitted HF and LF pulses with depth. We are in this invention mainly concerned with a situation where one transmits a LF/HF pulse complex with reception in the HF band only, but it is clear due to reciprocity that the probes also can be used to receive the LF band.